In this note, we apply the sub_sampling idea to the design of the chaotic secure communication system and analyze the Lorenz system and a kind of hyper chaotic system. The result obtained indicates that the security of the cryptosystem is not only determined by the dimension, but also related with the sampling interval. Then, we verify these two systems using the VWK (Volterra_Wiener_Korengerg) nonlinear test and surrogate data test methods.